A solid acid catalyst SO₃/γ-Al₂O₃ was prepared by adsorption of SO₃ gas onto the surfaces of activated γ-Al₂O₃. The structure and acidity of the catalyst were studied by XRD, NMR, FT-IR, TG-DTA and NH₃-TPD method. The results indicated that SO₃ reacted with activated γ-Al₂O₃ to form a small quantity of Al₂(SO₄)₃, and at the same time, the adsorption of SO₃ on the surface of γ-Al₂O₃ introduced two different kinds of Brönsted acid sites, which can be attributed to two different acid hydroxyl groups, including the bridging hydroxyl groups (S-OH-Al, giving rise to an ¹H resonance at ca. δ 4.3) and the terminal hydroxyl groups attached to S atoms (S-OH, giving rise to an ¹H resonance at ca. δ 11.2). The structure of Brönsted acid sites on the surface of catalyst SO₃/γ-Al₂O₃ has been proposed based on ¹H/²⁷Al TRAPDOR and FT-IR results. The acid strength of SO₃/γ-Al₂O₃ is stronger than that of zeolite HZSM-5, but still weaker than that of /γ-Al₂O₃, which is a conventional solid superacid catalyst.

Key words: SO₃/γ-Al₂O₃, solid acid catalyst, solid-state NMR, acid site structure